1. Field of the Disclosure
The present application relates to a control apparatus for a hybrid vehicle including both an internal combustion engine and an electric motor as drive sources of the vehicle.
2. Description of Related Art
There is known a hybrid vehicle (hereinafter, also simply referred to as vehicle) including both an internal combustion engine (hereinafter, also simply referred to as engine) and an electric motor as drive sources of the vehicle. The vehicle includes a storage battery. The storage battery supplies electric power to the electric motor, and is charged by using the output power of the engine.
In addition, when the rotation of an axle is transmitted to the electric motor, the electric motor generates electric power (that is, a generator generates electric power), and the storage battery is also charged with the generated electric power. That is, the kinetic energy of the vehicle is converted to electric energy, and the electric energy is recovered by the storage battery. This conversion of energy is also referred to as regeneration. When regeneration is performed, braking force (torque that decelerates a vehicle speed) of the vehicle, which is generated by the electric motor, is also referred to as regenerative braking force.
It is possible to improve the fuel consumption (fuel consumption rate) of the vehicle by recovering part of energy, consumed by the engine or the electric motor during acceleration and constant speed traveling of the vehicle, through regeneration during deceleration and storing the part of the energy in the storage battery. While the vehicle is traveling, the remaining amount of charge SOC (state of charge; hereinafter; also simply referred to as SOC) of the storage battery fluctuates.
When an increase and decrease in the remaining amount of charge SOC are repeated in any one of a state where the remaining amount of charge SOC is high and a state where the remaining amount of charge SOC is low, degradation of the storage battery is facilitated. For this reason, while the vehicle is traveling; the control apparatus for a vehicle keeps the remaining amount of charge SOC between a predetermined upper limit remaining amount of charge and a predetermined lower limit remaining amount of charge.
Incidentally, when the vehicle travels on a downhill section, the vehicle continues accelerating even when the engine and the electric motor generate no torque (driving force). Therefore, a driver of the vehicle releases the foot from an accelerator pedal and, where necessary, depresses a brake pedal to require braking force from the vehicle. At this time, the vehicle suppresses an increase in vehicle speed by the use of regenerative braking force and increases the remaining amount of charge SOC.
As the remaining amount of charge SOC increases, that is, as the amount of electric power that is charged in the storage battery increases; the distance that the vehicle is allowed to travel by using only the output power of the electric motor while the operation of the engine is stopped extends. Therefore, if the remaining amount of charge SOC is increased as much as possible within the range in which the remaining amount of charge SOC is lower than the upper limit remaining amount of charge when the vehicle travels on a downhill section, it is possible to further improve the fuel consumption of the vehicle.
However, if a downhill section is long, the remaining amount of charge SOC reaches the upper limit remaining amount of charge, so it is not possible to increase the remaining amount of charge SOC any more. Therefore; the effect of improvement in fuel consumption, which is obtained when the vehicle travels on a downhill section, increases as the difference between the upper limit remaining amount of charge and the remaining amount of charge SOC at the start point of the downhill section increases.
One of existing drive control apparatuses (hereinafter, also referred to as existing apparatus) increases the upper limit remaining amount of charge and decreases the lower limit remaining amount of charge when there is a downhill section having a predetermined altitude difference in a travel route. In addition, the existing apparatus gives a higher priority to traveling with the use of the electric motor than to traveling with the use of the engine such that the remaining amount of charge SOC approaches the decreased lower limit remaining amount of charge as much as possible by the time the vehicle enters the downhill section (see, for example, Japanese Patent Application Publication No. 2005-160269 (JP 2005-160269 A)). Therefore, with the existing apparatus, when a downhill section is included in a travel route, it is possible to improve the fuel consumption of the vehicle.
Generally, when the output power of the engine is small, the operation efficiency of the engine is low. For this reason, when the vehicle starts traveling or when the vehicle travels at a low speed, the control apparatus for a vehicle causes only the electric motor to generate output power while stopping the engine.
On the other hand, when the vehicle travels on a traffic congestion section, the vehicle travels at a low speed or the vehicle repeats traveling at a low speed and stopping. Therefore, when the vehicle travels on a traffic congestion section, the frequency at which the vehicle travels by using the output power of only the electric motor increases, and the amount of electric power that is recoverable through regenerative braking is not so large because the vehicle speed is low during regenerative braking. Therefore, the remaining amount of charge SOC decreases. In this way, if the remaining amount of charge SOC decreases in the case where the vehicle travels on a traffic congestion section and, as a result, the remaining amount of charge SOC reaches the lower limit remaining amount of charge, it is necessary to carry out forcible charging by using the output power of the engine, so there is a concern that the fuel consumption deteriorates.
The inventors studied this as follows. When there is a downhill section in a travel route, an actual remaining amount of charge is decreased in advance by decreasing a target remaining amount of charge, which is a target value of the remaining amount of charge of the storage battery, before entering the downhill section. Thus, it is possible to recover a larger amount of electric power in the downhill section. When a traffic congestion section is included in a travel route, an actual remaining amount of charge is increased in advance by increasing a target remaining amount of charge before entering the traffic congestion section. Thus, frequent forcible charging is prevented on the traffic congestion section.
However, for example, if the vehicle is traveling on a traffic congestion section at the time when the remaining amount of charge is intended to be decreased by decreasing the target remaining amount of charge in preparation for a downhill section, the vehicle is generally driven by using only the driving force of the electric motor without operating the engine. Therefore, the remaining amount of charge excessively decreases, with the result that forcible charging is performed and there is a concern that the fuel consumption deteriorates on the contrary. On the other hand, for example, if the vehicle is traveling on a traffic congestion section at the time when the remaining amount of charge is intended to be increased by increasing the target remaining amount of charge in preparation for a traffic congestion section, the frequency at which the engine is operated in a low load state having a low operation efficiency is increased in an attempt to increase the remaining amount of charge, and there is a concern that the fuel consumption deteriorates on the contrary.